docs: def proxy_for(self, obj):
          """Returns the ``Proxy`` for the target object, creating it if necessary.
  
          :param object obj: The object that will be doubled.
          :return: The mapped ``Proxy``.
          :rtype: Proxy
          """
  
          obj_id = id(obj)
  
          if obj_id not in self._proxies:
              self._proxies[obj_id] = Proxy(obj)
  
          return self._proxies[obj_id]